The present invention relates to controls for a hydrostatic transmission and more specifically relates to controls for effecting a positive neutral condition in the transmission when a control lever for the transmission is moved to a neutral position.
Hydrostatic transmissions of one known type include a reversible, variable displacement pump unit connected to a fixed or variable displacement motor unit such that the latter is caused to be driven at increasing speeds in forward and reverse respectively in response to a swashplate of the pump being increasingly angularly adjusted to opposite sides of a zero-displacement effecting neutral position.
The swashplates of such transmissions are normally adjusted by one double-acting or two single-acting hydraulic actuators, to and from which the flow of control fluid is controlled by a displacement control valve. Many of these transmissions, as exemplified by the transmission disclosed in U.S. Pat. No. 3,411,297 issued to Hann on Nov. 19, 1968, are controlled by displacement control valves actuated directly through a linkage connected to a manually operable control valve and including a feedback linkage coupled between the swashplate and the control valve such as to terminate flow to and from the swashplate actuator or actuators once the swashplate arrives at the position commanded by the control lever. The control linkages are constructed such as to place the valve in a neutral position connecting the swashplate actuator or actuators to sump when the control lever is in a corresponding neutral position and various provisions are made for adjusting the linkages such as to ensure that this correspondence in neutral positions will occur. Neutraling springs are provided in conjunction with the swashplate actuators such as to effect a zero-displacement condition in the pump in the absence of fluid pressure in the actuators.
Others of these transmissions, as exemplified by the transmissions respectively disclosed in U.S. Pat. No. 3,803,987, granted to Knapp on Apr. 16, 1974, and applicants' co-pending application Ser. No. 967,368 filed on Dec. 7, 1978, provide for pilot-operation of the displacement control valve. In the patented transmission, a control lever is connected to a fluid power spool and is operable to opposite sides of a neutral position to respectively cause the spool to force pilot pressure fluid to the opposite sides of a piston of a displacement control valve to effect shifting of the latter. When the spool is in a neutral position corresponding to the neutral position of the control lever, it acts to establish a float condition in the piston of the displacement control valve and neutraling springs act on both the spool and the piston to the end of having them positioned in respective neutral positions corresponding to the neutral position of the control lever. However, because there is considerable distance between the spool and the piston, there exists the danger that a malfunction may occur resulting in pressure fluid somehow becoming trapped in the pilot fluid passages extending between the spool and piston despite the fact that the control lever is placed in its neutral position.
As to the transmission disclosed in the aforementioned co-pending application, pilot operation of the displacement control valve is achieved electronically by providing an electric torque motor which is operative to effect a fluid pressure imbalance across a displacement control valve spool, which imbalance corresponds to the magnitude and direction of electrical command signals sent to the torque motor. A manually operable control lever acts on a variable potentiometer to produce increasing signals of opposite polarity respectively in response to the lever being increasingly displaced to opposite sides of a neutral position. The danger in this system is that the electric control circuitry may somehow malfunction to send a spurious control signal to the torque motor despite the fact that the control lever is in its neutral position.